1. Field of the Invention
The present invention relates to an electrophotographic recording apparatus with an improved measurement of a resistance of a circuit including a photoconductive drum, transfer roller, and a print medium sandwiched between the photoconductive drum and the transfer roller. The improved measurement of the resistance provides an optimum transfer voltage.
2. Description of the Related Art
In electrophotographic printers, the surface of a photoconductive drum is charged and the charged surface is then exposed to a print image. The print image forms an electrostatic latent image on the charged surface. The electrostatic latent image is developed with a toner into a toner image which in turn is transferred by a transfer roller to a print medium such as paper. The transferred toner image is subsequently fused. The transfer roller receives a transfer voltage from a high voltage power supply circuit and produces a Coulomb force which transfers the toner image to the print medium. The transfer efficiency depends on the conditions such as the size and thickness of the print medium and environment humidity and temperature. Therefore, when the print medium has been pulled in between the transfer roller and the photoconductive drum, the electrical resistance of a circuit including the print medium is measured so that the high voltage power supply circuit outputs a high voltage to run an optimum transfer current in accordance with the the resistance of the print medium.
The aforementioned conventional art suffers from the following drawbacks. The print medium is pulled in between the transfer roller and the photoconductive drum, and passes a transfer point where the print medium is sandwiched between the transfer roller and the photoconductive drum. The leading end portion of the print medium which is commonly referred to as "top margin" is subjected to a measurement of electrical resistance of the print medium. This area is outside of a print area of the print medium and usually lies over a very short distance in the direction of travel of the print medium. Therefore, when the print medium travels fast, i.e., a printing is performed at a high speed, the top margin passes the transfer point too soon before the voltage applied to the print medium and the current through the print medium become stable. In other words, after the application of a voltage to the print medium, the current flowing through the print medium requires some time before it reaches a stable value due to static capacitances of various parts. In order to address this drawback, one method has been developed in which a true value is determined by calculation based on an actually measured value. However, the actual resistance of the print medium is greatly affected by the electrical resistance values of the photoconductive drum and print medium, or static capacitance values. Combinations of these physical quantities further dictate the measurement of resistance of the print medium. Thus, a simple calculation does not provide a sufficiently accurate result but causes large errors.